1. Field of the Invention
The present invention relates to an optical pickup device used for optical information processing instruments such as CDROMs and DVDs, and more particularly to an optical pickup device having a structure for detecting a stable focus error signal free from the wavelength variation and positional variation of tracks of an objective lens to therefore provide a stable recording signal while involving less positional variation of optical parts and less degraded signal characteristics.
2. Description of the Prior Art
A hologram head which is employed as an optical head provided by simplifying a conventional optical system has been suggested in Japanese Patent Laid-open Publication No. sho63-229640.
In the hologram head as shown in FIG. 1, once a laser beam 111 is radiated from a light source 101 being a semiconductor laser, beam 111 transmits through a hologram 103 and then is incident to an objective lens 104. At this time, beam 111 transmitted through hologram 103 is divided into a zero order beam +first order beam and -first order beam. Among these beams, the zero order beam is to be solely utilized.
The beam having passed through objective lens 104 reaches an information recording/reproducing plane of a disc 105 to form a focus of light, and reflected beam reflected from disc 105 again transmits through objective lens 104 to be incident to hologram 103. The -first order diffracted beam 131 and +first order diffracted beam 132 diffracted by hologram 103 are respectively incident to two photodetectors 106 which are arranged in the vicinity of light source 101.
Photodetectors 106 are placed on a photodetector stand 107, of which rotation is adjustable.
Here, the reflected beam having passed through hologram 103 is in the form of having two conjugate focuses 171 and 172 that are placed at the front and rear sides of light source 101 with respect to the optical axis direction.
In other words, -first order diffracted beam 131 and +first order diffracted beam 132 reaching left and right photodetectors 106 are set to focus on spots provided further before or after respective photodetectors 106, which will be described in more detail with reference to FIGS. 2a, 2b and 2c.
When the information recording/reproducing plane of disc 105 is timely placed to the focus point of objective lens 104, as shown in FIG. 2b, left and right diffracted beams 131 and 132 exactly focus on their own positions, respectively. Accordingly, the diameters of diffracted beams 131 and 132 detected by photodetectors 106 are of the same size.
Also, when the information recording/reproducing plane, i.e., disc 105, is distant from objective lens 104, as shown in FIG. 2a, the diameter of left diffracted beam 131 detected by left photodetector 106 is increased in size while that of right diffracted beam 132 is decreased.
Contrarily, when the information recording/reproducing plane, i.e., disc 105, gets nearer to objective lens 104, the diameter of left diffracted beam 131 is decreased in size, and that of right diffracted beam 132 is increased.
Therefore, The focusing state between objective lens 104 and disc 105 can be perceived in view of the quantity of light of the diffracted beams accepted within light receiving areas 163 to 168 of both left and right photodetectors 106. Assuming that such a focus error value is denoted by Fe, Fe can be defined by a value which is obtained by subtracting the quantity of light accepted by light receiving element 164 from that accepted by light receiving element 167. That is, it is written as Fe=167-164 where 167 denotes the quantity of light accepted by light receiving element 167 and 164 denotes that accepted by light receiving element 164. Otherwise, the focus error value Fe can be given as the equation that Fe=(163+165+167)-(164+166+168).
Hologram 103 allows the focus of the diffracted beam to be formed onto any other place from that of the zero order beam when diffracting the beams radiated from light source 101. For this fact, unnecessary focusing on the information recording/reproducing plane is not performed when conducting the recording/reproducing operation to nor insert/record unnecessary playback signal.
In connection with the variation of the diffraction angle resulting from the wavelength difference of light source 101 in the above-described conventional structure, photodetectors 164 and 167 parallel to light source 101 in the radiating direction are employed. For this reason, the laser beam is moved along with the division direction even in the case of producing the wavelength variation, so that the structure hardly induces variation in the focus error signal, etc. However, due to this fact, it is disadvantageous as below.
First, because conventional hologram 103 is provided in a manner to have two conjugate focuses 171 and 172 on both sides of light source 101, the photodetectors are respectively required on both sides of light source 101 to interpose it between them.
Consequently, in the conventional structure, two photodetectors 106 are to be manufactured to make fixing of them onto photodetector stand 107 greatly fastidious because of the precisely symmetrical installation on the identical places. According to circumstances, it may be formed such that a large silicon substrate is perforated to prepare the light source in the hole formed. But this way has a problem of significantly heightening unit cost of the silicon substrate and, furthermore, requiring a new technique (Hybrid facilities) for fixedly installing the light source onto the center of the integrated photodetector.
Second, since conventional hologram 103 has two conjugate focuses 171 and 172 by -first order diffracted beam 131 and +first order diffracted beam 132, the optical axis-oriented distance from hologram 103 to reaching both photodetectors 106 should be always the same relative to the optical axis-oriented distance from hologram 103 to light source 101 so as to set two conjugate focuses 171 and 172 onto before and after the optical axis direction with respect to light source 101. Thus, it is very difficult to change in designing the distance relation among hologram 103, light source 101 and photodetectors 106.
On the other hand, Japanese Patent Laid-open Publication No. sho63-13134 describes another prior art of using the hologram.
The art is for detecting a focus error by astigmatism. As shown in FIGS. 3a, 3b and 3c, a focus error signal Fe is detected as follows by means of a photodetector 206 divided-by-four, EQU Fe=(263+266)-(264+265)
In this optical system, the beam is placed onto the center of photodetector 206 as shown in FIG. 3b when the objective lens is centrally arranged. However, if the objective lens is moved toward the inner circumference of the disc while moving from the center by tracing the tracks, the position of the beam is moved to the lower side of the center line of photodetector 206 as designated in FIG. 3a. Besides, when the objective lens is moved toward the outer circumference of the disc, the position of the beam is moved to the upper side of the center line of photodetector 206 as designated in FIG. 3c. Hence, when the objective lens is placed other than the center, the focus error signal involves variation as well as the sensitivity is changed.